The present invention relates to the field of film depositions by use of plasma and more particularly refers to a new technique and system for plasma enhanced chemical deposition (PECVD) wherein selective surfaces of tubular substrates may be treated to deposit thin films of a desired matter with a simple straightforward inventive system wherein one of the electrodes employed in the plasma system is conformed by the same substrate or workpiece without the need of bulky plasma reactors.
Plasma Enhanced Chemical Vapor Deposition (PECVD) is a process based in the use of an ionized gas generally named as plasma. The plasma is any gas in which a significant percentage of the atoms or molecules are ionized. Plasma is a state of matter distinct from common gases and it has unique properties. The term “ionized” refers to presence of one or more free electrons, which are not bound to an atom or molecule. The free electric charges make the plasma electrically conductive so that it responds strongly to electric, magnetic, and electromagnetic fields.
Different types of plasma may be formed by heating and ionizing a gas, stripping electrons away from atoms, thereby enabling the positive and negative charges to move more freely. The properties of plasma allow one to carry out a process, namely the PECVD, to deposit thin films from a gas state (vapour) to a solid state on some substrate such as a workpiece. Plasma deposition is generally carried out under RF (Radio frequency), AC (Alternating current) frequency or DC (Direct current) discharge between two electrodes where in-between place is filled with the reacting gases. The substrate is exposed to these reacting gases and the deposition results in a film chemically adhered to, or integrated to the surface of the substrate. The plasma is normally more positive than any object it is in contact with, as otherwise a large flux of electrons would flow from the plasma to the object. The voltage across the plasma and the objects it contacts is normally dropped across a thin sheath region. Ionized atoms or molecules that diffuse to the edge of the sheath region feel an electrostatic force and are accelerated towards the neighbouring surface. Thus all surfaces exposed to plasma receive energetic ion bombardment.
Several types of plasma reactors are known in the art and all of them are basically comprised of a bulky closed vacuum chamber with two electrodes installed therein. The electrodes are respectively connected to opposite electrical charges through respective connections from outside the closed vacuum chamber. A reactor may be operated by direct-current (DC) discharge which can be created between the two conductive electrodes, and may be suitable for deposition of conductive materials. It is also possible to excite a capacitive discharge by applying an alternating-current (AC) or radio-frequency (RF) signal between an electrode and the conductive walls of a reactor chamber, or between two cylindrical conductive electrodes facing one another. The kind of reactor will depend on the kind of pieces to be subject to treatment.
The chamber has several ports to receive the process gases and precursor matters necessary for the chemical reaction under either an electromagnetic, electrical, or RF field. The plasma is generated inside the vacuum chamber and the substrate is located into the vacuum chamber to be exposed to the plasma and for receiving the matter as a deposition to form the desired film cover or lining. Vacuum chambers may be small or large depending of the pieces that are to be inserted therein but, generally, bulky chambers are involved in order to have enough capacity to all kind of pieces. Always, the entire piece inside the chamber is exposed to the plasma and deposition will be done in all the exposed surfaces of the workpiece.
It is a frequent concern that certain bulky pieces can not be treated because no available plasma reactors are found and designing and manufacturing a special reactor for a determined type of workpiece may be not economically viable. Another concern is that plasma causes film deposition all over the surfaces of the workpiece into the vacuum chamber but, in some cases, deposition is desired for only some parts or surfaces of the substrate. For special works the deposition may be desired in only selective surfaces of the piece such as the inner surface of a tube, pipe or conduit, for example. If a pipe is introduced in the vacuum chamber all the surfaces thereof will be covered by the deposition film, while deposition may not be needed at the outer surface of the tube. Generally, the inner surface of a pipe is needed of treatment. A clear example is the recycling of used pipes or the protection of new pipes in any industry and particularly in the oil field. Considering the sizes of involved pipes no plasma chambers are easily available for that service.
Under the above circumstances it would be very convenient to have a new technique and system for carrying out a PECVD in tubular pieces, either of small and large sizes as well as for obtaining a deposition in only selective parts of the piece under treatment, without the need of a bulky system and stationary installations.